Paul MacLean, an American physician proposed that the human brain was in reality three brains in one; the reptilian brain, the limbic system and the neocortex.

The reptilian brain includes the brain stem and cerebellum. The term "Reptilian Brain" came from the fact that a reptile's brain is dominated by the brain stem and cerebellum which controls instinctive survival behaviour and thinking. In humans this part of the brain controls the muscles, balance and autonomic functions such as breathing and heartbeat.

The limbic system, also known as the "Mammalian Brain" is the source of our emotions and instincts (i.e. feeding, fighting, fleeing, and sexual behaviour). When this part of the brain is accessed, emotions are produced. MacLean observed that everything in the limbic system is either "agreeable or disagreeable." This means that survival is based upon the avoidance of pain (disagreeable) and the recurrence of pleasure (agreeable). The neocortex, also known as the cerebral cortex, is similar to the brain of higher mammals and controls higher-order thinking skills, reason and speech.

Also during the 1950s Roger Sperry and his colleagues conducted tests on people who had have two hemispheres of the brain surgically separated. This procedure involved severing the corpus callosum, the area of the brain used to transfer signals between the right and left hemispheres, and was used at the time to alleviate the worst symptoms of epilepsy.

As a result of the surgery, Sperry and his team were able to study the activity that occurred in each hemisphere of the brain in isolation while performing tests that were known to be dependent on specific hemispheres of the brain and demonstrated that the two halves of the brain each contain consciousness. In his words, each hemisphere is...

"a conscious system in its own right, perceiving, thinking, remembering, reasoning, willing, and emoting, all at a characteristically human level, and . . . both the left and the right hemisphere may be conscious simultaneously in different, even in mutually conflicting, mental experiences that run along in parallel." (Sperry, 1974)